Prefix de- used here in the sense of "thoroughly."
Totally 'frauded' coming out as defrauded.
De - is a Latin adjective and preposition. meaning "down from, off, concerning." Used as a prefix in English, as in defrost (1895), defuse (1943), decaffeination (1927), etc. Usually felt as meaning "down," but in Latin it could also be completive in intensive (cf. demerit), perhaps with a sense of "down to the bottom, totally." Also in de facto "in fact" (1602), which is usually contrasted with de jure "of right, according to law" (1611), both now used as adjectives in English.
Hope that answers your question Jeff.